With present day concern about air pollution, efforts are being made to burn coal or other solid fuel with a minimum of NO.sub.x and SO.sub.x in the combustion exhaust gases. In firing pulverized coal in the furnace of a steam generator, it is known that reducing the peak flame temperature will reduce the NO.sub.x formed. It is also known that firing with a deficiency of air (sub-stoichiometric or fuel rich) or with very little excess air (0-3%) will reduce flame temperature, thus minimizing the emission of SO.sub.x from the sulphur contained in the coal. The lower temperature encourages alkali material (in the coal itself or injected with the coal) to react with the sulphur. Also, with lower temperature, more reactive sulphur compounds are formed.